Vehicle systems can have multiple propulsion-generating vehicles that include communication devices for the vehicles to communicate with each other. The vehicles can communicate with each other to coordinate individual vehicle movements so that the entire vehicle system moves along routes. For example, locomotives can communicate with each other using wireless communications so that one locomotive can remotely direct the throttle settings, brake settings, etc., of the other locomotives in the same rail vehicle system.
The locomotives may send messages (also referred to as signals) back-and-forth to ensure that messages commanding changes in throttle settings and/or brake settings are sent and that messages indicating the changes were received are sent. These vehicle systems typically employ a contention scheme to prevent overlapping interference between the different messages. For example, some locomotives use a slotted aloha protocol to allow multiple trains containing multiple locomotives to use the same frequency channel for communicating messages between the locomotives in the same train. This protocol divides the times at which different locomotives can communicate over the channel into time slots that are accessed on a prioritized but randomized basis.
Once a lead locomotive from a one train seizes the channel, that lead locomotive and the remote locomotives that are controlled by that lead locomotive have exclusivity of the channel for a defined period of time. This channel exclusivity allows for data communication between the lead locomotive and the remote locomotives to occur without interference from communication between other locomotives using the same channel.
This scheme relies on all locomotives within radio range to receive a radio protocol preamble from other transmitters on the channel to force the locomotives to hold off channel access for a preset period of time. In the current implementation, this occurs using Frequency Shift Keying (FSK) modulation. Other modulation schemes cannot be used at the same time on the same channel due to the incompatibility of the modulation schemes. For example, these locomotives or other devices cannot use Shaped Offset Quadrature Phase Shift Keying (SOQPSK) on the same channel and at the same time as the two modulation methods are mutually incompatible and neither modulation method would hold off transmitting for the other method.